In many separation processes, and in distillation processes in particular, essentially undefined solid residues are obtained after isolation as a bottom product, which have to be removed. In the interests of ease of handling, these solid residues are not isolated completely during distillation, but are expelled from the distillation process in the form of a solution in materials still capable of evaporation, or in specially added solvents. The residue solution requires further work-up in order to obtain the evaporable materials and to recover any solvents used. Work-up is generally carried out in such a way that the residue solution is heated to the evaporation temperature for the evaporable materials and solvents. The evaporating substances are drawn off and a solid, free-flowing residue is obtained which is conveyed to a further disposal operation such as incineration or dumping. A problem from the process technology angle is that while the evaporable materials or solvents are being evaporated, the residue solution changes from a liquid state via a state with a viscous, sticky consistency to the solid residue. If evaporation takes place batchwise in stirred apparatus, the transition may be observed on the basis of the greatly increasing stirrer power required during the passage through the viscous-sticky state. During the viscous-sticky state, moreover, the heat transfer to the residue mass (introduction of the heat of evaporation) and the mixing required to produce new surfaces with a high concentration of evaporable substances is greatly impeded.
An example of such distillation residues are residues from distillation during the production of isocyanates, particularly toluene diisocyanate (TDI), a chief component for the production of polyurethanes. The production of isocyanates takes place by phosgenation of corresponding amines, the reactants being reacted in a solvent, usually ortho-dichlorobenzene. The reaction takes place with a yield of approx. 96 to 98%, with primarily isocyanate polymers being formed as by-products.
In order to obtain pure isocyanates, the crude isocyanate solution obtained by phosgenation is distilled in several partial stages. A residue solution which, in the case of TDI production, contains about 5 to 20% of polymeric TDI, is obtained as a bottom product of the distillation columns. Evaporable proportions of TDI and solvents have to be evaporated from the residue solution to obtain a solid, free-flowing residue.
According to U.S. Pat. No. 2,889,257, it has already been proposed to work up residue solutions from isocyanate production by introducing the residue solution into a stirred vessel containing inert, high-boiling hydrocarbons at a temperature of 200.degree. to 350.degree. C., the evaporable proportions of the residue solution evaporating and being drawn off, and solid residue particles being formed in the hot hydrocarbon oil. The hot hydrocarbon oil is pumped around between the stirred vessel and a heat exchanger, the solid particles being expelled by gravimetry from a partial stream. As a result of this inherently complex process, the transition via the viscous-sticky state of the residue solution is avoided, but the process requires elaborate apparatus and, in addition, requires the work-up of the hydrocarbon oil at certain intervals.
According to a more recent proposal in European Patent 548,685, work-up takes place batchwise in a stirred vessel in which is placed a charge of 5 to 20% of bitumen based on the quantity of residue solution at a temperature of 150.degree. to 280.degree. C., to which the residue solution is then introduced continuously, the evaporable constituents of the residue solution being drawn off at a pressure of 2 to 30 mbar. When the charge has ended, a solid residue is obtained in the form of a crumbly, free-flowing mass. The bitumen charge evidently fulfills the several functions. The stirred vessel is initially provided with a sufficient heat capacity to evaporate the evaporable constituents of the residue solution, so that temperature variations are avoided. During the initial introduction of residue solution, the bitumen ensures a rapid distribution of the residue solution in the stirred vessel. Caking of solid residue on the stirrer and container walls is avoided. Moreover, the formation of small residue crumbs is promoted. However, the problem of the transition via the viscous-sticky state, which requires a substantially increased stirring power, is only slightly reduced. Moreover, a disadvantage is the batchwise mode of operation requiring large numbers of operatives, which mode can be designed as a quasi continuous operation only by the alternating operation of several stirred vessels.